1. Field of the Invention
The present invention relates to a shadow mask type color cathode ray tube used for a television receiver, a computer display, and the like.
2. Description of the Related Art
FIG. 8 is a cross-sectional view showing one example of a conventional color cathode ray tube. The color cathode ray tube 1a in FIG. 8 includes a substantially rectangular-shaped face panel 2 having a phosphor screen 2a formed on its inner surface, a funnel 3 connected to the rear side of the face panel 2, an electron gun 4 contained in a neck portion 3a of the funnel 3, a shadow mask 6 facing the phosphor screen 2a inside the face panel 2, and a long frame 7 for fixing the shadow mask 6. Furthermore, in order to deflect and scan electron beams, a deflection yoke 5 is provided on the outer periphery of the funnel 3.
The shadow mask 6 plays the role of selecting colors with respect to three electron beams emitted from the electron gun 4. The shadow mask 6 is a flat plate in which a number of apertures, through which electron beams pass, are formed by etching. xe2x80x98Axe2x80x99 shows a track of the electron beams.
The long frames 7 fixes the shadow mask 6, and a pair of short frames 8 are fixed to the longitudinal ends of the long frames 7. The pair of long frames 7 and the pair of short frames 8 form a frame structure. This frame structure and a shadow mask 6 fixed to the frame structure compose a shadow mask structure 9.
Plate-shaped spring-attaching members 21 are adhered to the pair of long frames 7, and spring members 10 are fixed to these spring-attaching members 21. Plate-shaped spring-attaching members 11 are adhered to the pair of short frames 8, and spring members 12 are adhered to the spring-attaching members 11.
The shadow mask structure 9 is fixed to the face panel 2 by fitting attaching holes 10a of the spring members 10 with pins 13 provided to the top and bottom of the inner surface of the face panel 2, and by fitting the attaching holes 12a of the spring members 12 with pins (not shown) provided to the right and left of the inner surface of the face panel 2.
In a color cathode ray tube, due to the thermal expansion of the shadow mask 6 caused by the impact of the emitted electron beams, the apertures for passing electron beams are displaced. Consequently, a doming phenomenon occurs. That is, the electron beams passing through the apertures fail to hit a predetermined phosphor correctly, thus causing unevenness in colors. Therefore, a tensile force to absorb the thermal expansion due to the temperature rise of the shadow mask is applied in advance, and then the shadow mask 6 is stretched and held to the long frames 7. When the shadow mask 6 is stretched and held as mentioned above, it is possible to reduce the displacement between an aperture of the shadow mask 6 and phosphor stripes of the phosphor screen 2a even if the temperature of the shadow mask 6 is raised.
However, the conventional color cathode ray tube described above suffered from the following problem. When an electron beam hits the stretched shadow mask 6, the shadow mask 6 is expanded by heat and its tensile force is reduced. Thereby, the internal moment of the shadow mask structure 9 changes and the balance changes as well. Due to the change in the balanced state, a distance 23 (q-value) between the apertures of the shadow mask 6 and the phosphor screen 2a is deviated, that is, the shadow mask 6 is displaced to recede from the phosphor screen 2a in the axial direction. This will prevent electron beams from hitting a desired position of the phosphor, which will lead to unevenness in colors. With respect to unevenness in colors, the displacement of the shadow mask 6 to recede from the phosphor screen 2a in the axial direction may be more unfavorable in general than displacement to approach the phosphor screen 2a. 
It is an object of the present invention to provide a cathode ray tube that can solve the problems of conventional techniques. Such a cathode ray tube can suppress a shadow mask from being displaced in an axial direction with respect to a phosphor screen and can prevent unevenness in colors.
To achieve the above object, a color cathode ray of the present invention comprises a pair of long frames, a pair of opposing short frames that are fixed to the pair of long frames and support the long frames, and a shadow mask fixed in a state applied with a tensile force to the pair of long frames, wherein the short frames have substantially-triangular bent parts formed to protrude toward the shadow mask. Since such a cathode ray tube can decrease an internal moment of the shadow mask structure, the displacement of the shadow mask in a direction to recede from the phosphor screen surface of the color cathode ray tube can be suppressed and the q-value deviation also can be suppressed even if the shadow mask is expanded by heat generated by an impact of electron beams.
The substantially-triangular bent parts of the color cathode ray tube comprise neutral axes at crests protruding toward the shadow mask and the neutral axes are located above a surface of the shadow mask. Accordingly, the shadow mask approaches the phosphor screen of the color cathode ray tube when it is expanded by heat, providing effects in correcting unevenness in colors.
The substantially-triangular bent parts of the color cathode ray tube form recesses having a width dimension in a range from ⅙ to xc2xd of the maximum length in the longitudinal direction of the short frames. Accordingly, sufficient effects in correcting unevenness in colors will be secured, and also the productivity is improved since the color cathode ray tube is less deformed by heat in the production process and the accuracy of its q-value is stabilized.
The substantially-triangular bent parts of the color cathode ray tube may have circular corners with an outer radius of curvature of at least 15 mm. Accordingly, excessive concentration of stress at the corners can be prevented so as to secure sufficient rigidity.
Additionally, support-adjusting members are fixed to the short frames by extending across the recesses formed by the substantially-triangular bent parts. Accordingly, the change of an inner moment can be decreased, and moreover, the short frames will have improved rigidity. Since the improved rigidity serves to increase the cross-sectional second moment, the cross-section area of the steel material used for the short frames can be decreased. Displacement of the shadow mask in the axial direction with respect to the phosphor screen of the color cathode ray tube is suppressed at a time of impact of electron beams.
The support-adjusting members have a thermal expansion coefficient that is bigger than that of the short frames, which can prevent plastic deformation of the shadow mask during a heat treatment step, and also suppress displacement in the axial direction at a time of operation of the color cathode ray tube.